date in MongoDB: when inserting Date objects into Mongo database, the date becomes 1 day earlier than itself

My date string format is like this: Jan 2, 2012
After the Instant.parse() method, instant instance becomes the date of Jan 1, 2012, which is 1 day earlier, why? If the original date string is jan 1, 2012, the Instant will be the date of Dec 31, 2011.

I'm using MongoDB to store these dates. I've tested and it shows when formatting date string->instant there's no mistake.
But when I insert this Date type object into MongoDB, the date string in the MongoDB becomes 1 day earlier., why?